1. Field of the Invention
The present invention relates to a boring apparatus or a magnetic base drill unit, and particularly to a light-weight and small sized boring apparatus.
2. Description of the Prior Art
Various kinds of boring apparatus as magnetic base drill units have already been proposed, which comprise a drill unit provided with an annular hole cutter, a magnetic base for causing the drill unit to be magnetically adhered onto a workpiece for fixing, a feed motor for automatically feeding the drill unit to the workpiece, and the like. A magnetic base drill unit has also been proposed, wherein the drill unit is restored to its initial position after the hole cutting work by the drill unit is completed.
For instance, in the specification of U. S. Pat. No. 4,789,274 issued on Dec. 6, 1988 and assigned to the same assignee as the present invention, a technique is described for enhancing the efficiency of the hole cutting work by making the restoring (rise) speed of the drill unit after the termination of the hole cutting work faster than the feed (fall) speed of the drill unit during the hole cutting work. Specifically, in the magnetic base drill unit described in the above U.S. Pat. No., rotation of the feed motor is transmitted to the drill unit through a reduction gear during the hole cutting work, while it is transmitted to the drill unit without through the reduction gear after the termination of the hole cutting work.
The feed mechanism of such boring apparatus is provided with a clutch in the power transmission device between the feed motor and the manual feed mechanism, so that the transmission of power from the feed motor to the feed mechanism can be released by letting out the clutch, thereby enabling the drill unit to be risen or fallen by using a manual handle.
In the U.S. Pat. No. described above, the drill unit is provided with a rack in the rise and fall direction thereof, and the gear of the final stage of a reduction gear train reducing the rotation of the feed motor is engaged with the rack, thereby constituting the feed unit of the drill unit A manual handle is provided on the supporting shaft of one of gears constituting the reduction gear train, and a pair of toothed halves (17A and 17B in the said specification) are arranged on the shaft so that each half is opposed to the other in the axial direction thereof, with one half being fixed to the shaft and the other half being fixed to the frame of the boring apparatus. The pair of toothed halves from the clutch. And, by sliding the shaft in the axial direction thereof, engagement and disengagement of the halves of the toothed halves are performed, thereby performing the transmission and release of power.
In addition, in the U.S. Pat. No. mentioned above, a worm is attached to the feed motor to provide power transmission to the worm wheel disposed on the end portion of the shaft of the manual handle. Accordingly, the feed motor is attached so that its rotating shaft is perpendicular to the shaft (rotating shaft) of the manual handle which is supported horizontally, or to the rise and fall direction of the drill unit.
The above described prior art had the following problems.
(1) Since the halves of the pair of toothed halves constituting a clutch are arranged so that they are orienting to the axial direction of the manual handle and, it is needed to slide the manual handle in the axial direction thereof for the engagement/disengagement of the toothed halves, the frame of the boring apparatus becomes large-sized in the axial direction of the manual handle. PA1 (2) Since the rotation of the feed motor is transmitted by using a worm and a worm wheel, it is required to install the feed motor so that the rorating shaft of the feed motor matches the rise and fall direction of the drill unit, so it is difficult to place the feed motor in the casing of the boring apparatus. Since the feed motor is separately provided outside the casing, the construction of the means for installing the feed motor is complicated and the boring apparatus becomes large-sized.